1. Field of the Invention
The present invention relates to a projection-type image display apparatus comprising a plurality of cathode-ray tubes for displaying different monochromatic images on the respective picture planes, and projecting the images on the screen.
2. Description of Related Art.
A schematic plan view of the optical system of a projection-type image display apparatus is shown in FIG. 1. In front of the screen 3 cathode-ray tubes (hereinafter referred to as "the CRTs") 11, 12, 13 are arranged in juxtaposition. The CRT 12 is located on one side of the central CRT 11 and the CRT 13 on the other side thereof. The picture planes (not shown) of the CRTs 11, 12, 13 are in opposed relation to the screen 3. The CRT 11 is adapted to illuminate in monochrome of green (G), the CRT 12 in monochrome of red (R), and the CRT 13 in monochrome of blue (B), so that a monochromatic image is displayed on the picture planes of the CRTs 11, 12, 13 respectively.
The projection-type image display apparatus of this type generally comprises a CRT for displaying a green (G) image arranged at the center, a CRT for displaying a red (R) image on one side, and a CRT for displaying a blue (B) image on the other side thereof. Projection lenses 21, 22, 23 are arranged in front of the CRTs 11, 12, 13, respectively. The optical axes of the projection lenses 22, 23 have a convergence angle 40 and are positioned inward of the screen 3.
Now, the operation of this projection-type image display apparatus will be described.
When images of green (G), red (R) and blue (B) are displayed separately on the respective picture planes of the CRTs 11, 12, 13, each image is projected on the screen 3 in the form enlarged by the projection lenses 21, 22, 23. A full-color image synthesized from the images of green (G), red (R) and blue (B) is thus displayed on the screen 3 as an image available for observation.
This projection-type image display apparatus has a convergence angle 40 between the optical axis 41 of green (G) (control CRT 11) and the optical axis 42 of red (R) (CRT 12), as shown in FIG. 1. As a result, when a rectangular raster of the same shape as the green (G) raster displayed on the picture plane of the CRT 11 is displayed on the picture plane of the CRT 12 for displaying a red (R) image, the image projected on the screen 3, as shown in FIG. 2, is such that the green (G) raster 51 is rectangular while the red (R) raster 52 is distorted to a trapezoid as shown by dashed line. FIG. 2 shows a shape of the red raster 52 projected on the screen as viewed from the user. Also, the blue (B) raster is distorted in a laterally inverted shape as compared with the red (R) raster 52. This is attributable to the presence of the convergence angle 40 which results in different magnifications of the images projected on the screen 3 because of different screen positions.
To obviate this problem, the CRTs 12, 13 for displaying red (R) and blue (B) images respectively have an auxiliary yoke 8 mounted thereon, as shown in FIG. 1. The auxiliary yoke 8 generates a magnetic field for securing the same shape as the raster projected from the central CRT 11 onto the screen 3 in order to correct the magnifications of the images as described above. FIG. 3 is a block diagram showing a circuit for correcting the raster shape by supplying current to the auxiliary yoke 8. An input terminal 200 for supplying a raster shape correction voltage is connected to an input terminal of an amplifier 7, the other input terminal of which is grounded through a resistor 92. The output terminal of the amplifier 7 is grounded through a series circuit including the auxiliary yoke 8 and a resistor 9, and the connecting point between the auxiliary yoke 8 and the resistor 9 is connected to the other input terminal of the amplifier 7 through a resistor 91.
Now, the operation of this raster shape correction circuit will be explained. The input terminal 200 is impressed with a sawtoothed-shaped voltage in horizontal period, as shown in FIG. 4, for correcting the width in horizontal direction (hereinafter referred to as the horizontal width), a parabolic waveform voltage in horizontal period, as shown in FIG. 5, for correcting the horizontal linearity, a sawtoothed-shaped voltage in vertical period, as shown in FIG. 6 for correcting the inclination of the vertical lines, a parabolic voltage in vertical period, as shown in FIG. 7, for correcting the arcuate distortion of vertical lines, a keystone correction voltage, as shown in FIG. 8, for correcting the trapezoidal distortion of the raster, and a raster shape correction voltage combined with a regulation voltage, etc. including a DC voltage for adjusting the static convergence. Thus, the raster shape correction voltage is amplified by the amplifier 7, and a current flows in the auxiliary yoke 8. The shape of the raster displayed on the picture planes of the CRTs 12, 13 is corrected in such a manner that the trapezoidal red (R) raster 52 shown in FIG. 2 becomes the same shape as the green (G) raster 51 on the screen 3. Although the description concerns the correction along the horizontal direction alone, a similar correction is possible using a similar circuit along the vertical direction, whereby the same shape is secured for the rasters 51 and 52.
In the above-mentioned system, when the static convergence on the screen is displaced by the effect of earth magnetism or the like, the static convergence regulation voltage containing a DC voltage described above is changed to adjust the static convergence at the central position of each raster of green (G), red (R) and blue (B). Nevertheless, the problem remains that even when the static convergence is adjusted at the central position of the screen, a convergence displacement occurs at the peripheral portions due to the presence of the convergence angle described above.